Display panels employing a self-emitting emitting element, as represented by an organic EL (Electro Luminescence) display panel or an FED, have a promising future as a candidate for a flat display panel which can match up against liquid crystal displays.
FIG. 19(a) and FIG. 19(b) show structures of a blue-color emitting organic EL element which was introduced in the SID 97 DIGEST pp. 1073–1076. FIG. 19(a) is a cross sectional view showing a structure of a conventional blue-color emitting organic EL element 311, and FIG. 19(b) is a structural formula of an emitting layer 307 of FIG. 19(a).
The blue-color emitting EL element 311 has a structure in which a transparent anode (transparent electrode) 302 such as ITO is formed on a glass 301, and an organic multi-layered film 304 is formed thereon. On the organic multi-layered film 304 is formed a cathode 303 such as Al. Among several structures of the organic multi-layered film 304 the foregoing publication adopts a structure in which a hole injecting layer 305, a hole transporting layer 306, an emitting layer 307, and an electron transporting layer 308 are stacked on the anode 302. Further, the foregoing publication discloses a structure for realizing full-color display by subjecting the monochromatic blue color luminescence to color-conversion with the use of a color-conversion filter. The emitting layer 307 has such a structure with the structural formula (biphenyl (DPVBi: provided by Idemitsu Kousan Co., Ltd.)) as shown in FIG. 19(b).
FIG. 20 is a structure of a conventional emitting organic EL element of three colors R, G, and B, as taught by NEC TECHNICAL JOURNAL Vol. 51 No. 10/1998 pp.28–32 (published date: Oct. 23, 1998), showing a cross sectional view of a pixel structure of the emitting organic EL element of three colors R, G, and B. Note that, in FIG. 20, structural elements which are analogous to those shown in FIG. 19(a) are given the same reference numerals. On a glass 301 is formed a transparent anode 302 such as ITO (the structure is upside down in FIG. 20), and an organic multi-layered film 304 is formed thereon. On the organic multi-layered film 304 is formed a cathode 303 such as Al. Among several structures of the organic multi-layered film 304, the foregoing publication adopts a structure in which a hole transporting layer 306, an emitting layer 307, and an electron transporting layer 308 are stacked on the anode 302. Further, in this publication, a hole injecting layer (not shown) and the hole transporting layer 306 are made of an aromatic amine material. That is, Further, in the emitting layer 307, a red emitting layer 307R is made of a G (green) emitting material as a host doping a pigment DCM for a red (R) laser, and a green emitting layer 307G and a blue emitting layer 307B are made of an aromatic amine material. The electron transporting layer 308 is made of a metal complex material.
Various other materials for the organic multi-layered film 304 can be found in many Japanese Unexamined Patent Publications, including, for example, Tokukaihei 5-70773 (published date: Mar. 23, 1993), Tokukaihei 5-198377 (published date: Aug. 6, 1993), and Tokukaihei 6-172751 (published date: Jun. 21, 1994).
FIG. 21 is a perspective view showing a structure of a simple-matrix-type EL panel (EL display panel) using such an organic EL element. Specifically, a plurality of anodes 2 extending in one direction and the cathodes 3 extending in a direction orthogonal to the anodes 2 are formed on a glass 1 with the organic multi-layered film 4 therebetween, thus making the simple-matrix-type EL panel, with the areas of intersections of the anodes 2 and the cathodes 3 making up pixels.
FIG. 22 is a graph showing a relation between a voltage between the cathode 303 and the anode 302 (cathode-anode voltage) and a current through the emitting layer 307 in the organic EL element as shown in FIG. 19 and FIG. 20. Also, FIG. 23 is a graph showing a relation between a current through the emitting layer 307 and luminance. As shown in FIG. 23, in the organic EL element, the current through the emitting layer 307 and the luminance are nearly proportionally related, which, however, is not the case for the relation between the cathode-anode voltage and the current through the emitting layer 307, which varies depending on such factors as temperature, as shown in FIG. 22.
Thus, the organic EL element is preferably driven by controlling current, rather than voltage, to stabilize luminance. As such is the case, the column (data) driver circuit for driving the organic EL element preferably has a structure as shown by a current-controlling driver circuit 116 in FIG. 24. FIG. 24 is a block diagram showing a structure of the matrix-type EL panel.
Here, the current-controlling driver circuit 116 is structured such that the voltage generated by the column driver circuit 112 is converted to current by a current control circuit (variable constant current circuit) 115. Apart from the question as to whether such a structure is preferable or not, the use of a current-controlling driver circuit is nonetheless preferable in the organic EL element.
In this structure, the current-controlling driver circuit 116 is connected to the anodes 2 of the EL panel 110 (as shown in FIG. 2), and a row (scanning) driver circuit 111 is connected to the cathodes 3, thus structuring the matrix-type EL panel. Note that, the column driver circuit 112 has a structure wherein a shift register 113 receives data according to luminance of respective colors R, G, and B (red, green, and blue), which are then transferred by a clock CLK and held in a sample hold circuit 114 by a data hold timing pulse LP, and a current is outputted from the current control circuit 115 based on this data.
A driving method of such a matrix-type EL panel of a simple matrix structure is disclosed in PIONEER R&D Vol. 8 No. 3, pp.41–49 (published date: Dec. 31, 1998). The following describes a driving method of the simple-matrix-type EL panel with reference to FIG. 25. FIG. 25 is a circuit diagram showing a driving circuit of the simple-matrix-type EL panel.
The potential of a selected row (scanning) electrode K2 is dropped to a GND potential, and the other row electrodes are set to a specific potential (here, about 10 V). Display is carried out in such a manner that constant currents are flown through column (data) electrodes A2 and A3 of their respective pixels E2.2 and E2.3 to be displayed, and the column electrodes which correspond to pixels not to emit light are set to an open state.
Further, in order to carry out multi-tone display on pixels, the current supplied from the column electrode is controlled according to a tone level to be displayed on the pixel. This current control is carried out by (1) a current value modulation tone control method, in which the intensity of the current outputted to the column electrodes (anodes 2 in FIG. 24) from the current control circuit 115 is changed according to the luminance of pixels to be displayed, and (2) a pulse width modulation tone control method, in which a supply time of a current is changed according to the luminance of pixels to be displayed, while holding the currents outputted to the column electrodes at a constant level.
Meanwhile, a structure of an active-matrix-type EL panel using diodes is shown, for example, in Tokukaihei 10-268798 (published date: Oct. 9, 1998).
FIG. 26 shows an equivalent circuit of a pixel of the EL panel of this publication. FIG. 26 is a circuit diagram showing an equivalent circuit of the active-matrix-type EL panel using diodes. A pixel 12 includes an organic EL element 13, an additional capacitance (auxiliary capacitance) 14, an additional resistance 15, and an MIM (Metal Insulator Metal) diode 16 as the pixel driving element.
The pixel 12 of the EL panel has a structure as shown in FIG. 27(a) and FIG. 27(b), in which FIG. 27(a) is a plan view showing the structure of the pixel 12 of FIG. 26, and FIG. 27(b) is a cross sectional view, taken along the line A—A of FIG. 27(a). The MIM diode 16 has a stacked structure of a cathode electrode 21 made of tantalum, an insulating film 22 made of a silicon oxide film, and an anode electrode 23 made of chrome, and is formed on an insulating substrate 31. The additional resistance 15 is made up of a wiring layer which is formed on the insulating substrate 31, making up a portion of the anode electrode 23 of the MIM diode 16 extending on the insulating substrate 31.
The additional capacitance 14 is composed of electrodes 41 and 42 opposing each other, and an insulating film 43. The electrode 41, made of tantalum, is formed on the insulating substrate 31. On the electrode 41 is formed the electrode 42, made of chrome, via the insulating film 43 made of a silicon oxide film. Further, the electrode 42 is connected to the wiring layer making up the additional capacitance 15. Here, the electrode 41 is formed simultaneously with the cathode electrode 21 of the MIM diode 16, and the electrode 42 is simultaneously formed with the anode electrode 23 of the MIM diode 16, and the insulating film 43 is simultaneously formed with the insulating film 22 of the MIM diode 16.
The organic EL element 13 has a stacked structure of an anode 51 made of a transparent electrode such as ITO, a hole transporting layer 52, an emitting layer 53, an electron transporting layer 54, and a cathode 55 made of aluminium alloy. The hole transporting layer 52, the emitting layer 53, and the electron transporting layer 54 are made of organic compounds. The cathode 55 is formed on the electrode 42 making up the additional capacitance 14, via the insulating film 56 made of a silicon oxide film. Further, the cathode 55 is connected to the anode electrode 23 of the MIM diode 16 via a contact hole 57 which is formed through the insulating film 56. The anode 51 is connected to the electrode 41 of the additional capacitance 14 via a contact hole 58 which is formed through the insulating film 43 and the insulating film 56.
A structure of the foregoing EL panel and a driving system of the EL panel is shown in FIG. 28. FIG. 28 is a block diagram showing a structure of the active-matrix-type EL panel using diodes.
An EL display device 84 includes an EL panel 81, a gate driver 82, and a drain driver (data driver) 83. On the EL panel 81 are disposed gate wires (scanning lines) G1, . . . , Gn, Gn+1, . . . , Gm, and drain wires (data lines) D1, . . . , Dn, Dn+1, . . . , Dm. The gate wires G1 through Gm and the drain wires D1 through Dm are orthogonal to each other, and the areas where they cross each other make up pixels 12. That is, the EL panel 81 is made up of the pixels 12 which are disposed in a matrix pattern. The gate wires G1 through Gm are connected to the gate driver 82 for receiving gate signals (scanning signals). Further, the drain wires D1 through Dm are connected to the drain driver 83 for receiving data signals.
Here, the gate wires G1 through Gm are made up of cathode electrodes 21 of the MIM diode 16. Further, the drain wires D1 through Dm are made up of electrodes 41 of the additional capacitance 14 extending on the insulating substrate 31 (FIG. 27(a) and FIG. 27(b)).
The following describes a driving method of the EL panel 84 with reference to FIG. 26 through FIG. 28. By controlling the voltage of the gate wire Gn so that the voltage between the gate wire Gn and the drain wire Dn becomes higher than the threshold voltage of the MIM diode 16, the MIM diode 16 becomes conducted. This sets off charging the electrostatic capacitance of the organic EL element 13, and the additional capacitance 14 by the data signal applied to the drain wire Dn, thus applying the data signal to the pixel 12. This data signal drives the organic EL element 13, and the organic EL element 13 emits light as a result.
On the other hand, when the voltage of the gate wire Gn is controlled so that the voltage between the gate wire Gn and the drain wire Dn becomes lower than the threshold voltage of the MIM diode 16, the MIM diode 16 becomes non-conducted. In this case, the data signal which had been applied to the drain wire Dn up to this time is held in the form of a charge by the electrostatic capacitance of the organic EL element 13, and the additional capacitance 14. By thus supplying data signals to be applied to the pixels 12 to the drain wires D1 through Dm, and by controlling the voltages of the gate wire G1 through Gm, any data signal can be held by the pixels 12. The organic EL element 13 can be driven continuously, i.e., can emit light, until the MIM diode 16 becomes non-conducted again.
Further, a structure of the active-matrix-type EL panel using a FET (Field Effect Transistor), or, in particular, a thin film transistor (TFT) is shown, for example, in Tokukaihei 8-234683 (published date: Sep. 13, 1996).
An equivalent circuit of the EL panel as disclosed in this publication is shown in FIG. 29. FIG. 29 is a circuit diagram showing an equivalent circuit of pixels of the EL panel of the active-matrix-type EL panel using TFTs. A two-dimensional structure of pixels of the EL panel is as shown in FIG. 30. FIG. 30 is a plan view of pixels of the active-matrix-type EL panel using TFTs.
Each pixel 212 of the EL panel includes two TFTs 213 and 214, a memory capacitor 215, and an organic EL element 216. The source of the TFT 213 is connected to a source bus (column electrode, source line 152), and the gate of the TFT 213 is connected to a gate bus (row electrode, gate line 151). To the drain of the TFT 213 are connected in parallel one of the terminals of the memory capacitor 215 and the gate of the TFT 214. The other terminal of the memory capacitor 215 and the source of the TFT 214 are connected to a ground pass 153, and the drain of the TFT 214 is connected to the anode (EL anode layer 418) of the organic EL element 216. The cathode of the organic EL element 216 is connected to a negative power source (not shown).
The TFT 214 and the memory capacitor 215 of the EL panel as shown in FIG. 30 has cross sectional structures as shown in FIG. 32 and FIG. 33, respectively. FIG. 32 is a cross sectional view taken along the line B—B of FIG. 30, and FIG. 33 is a cross sectional view taken along the line C—C of FIG. 30. The TFT 214 and the memory capacitor 215 are fabricated as follows.
A polysilicon layer 411 is deposited on a transparent insulating substrate 410 made of a material such as crystal or low-temperature glass, and the polysilicon layer 411 is patterned in the form of an “island” by photolithography. Then, an insulating gate material 412 such as silicon dioxide is deposited in the thickness of about 1000 Å on the surfaces of the polysilicon layer 411 of the island shape and the insulating substrate 410.
Then, a polysilicon layer 413 made of amorphous silicon is deposited on the gate insulating layer 412, and is patterned by photolithography on the polysilicon island so that source and drain areas are formed in the polysilicon area after ion implantation. The ion implant is conducted with an N-type dopant, for which arsenic is used. The polysilicon gate electrode 413 also serves as a base electrode 413a of the capacitor 215. The gate bus 414 is made of metal silicides such as tungsten silicide (WSi2) and is patterned.
Thereafter, an insulating layer 415 such as silicon dioxide is deposited over the entire surface of the device. A portion of the insulating layer 415 is used to form contact holes 416a and 417a, etc., so as to provide a junction in the thin film transistor. An electrode material 416, which is provided in contact with the source area of the TFT 214, also makes up an upper electrode 416b of the capacitor 215. The source bus and the ground bus are also formed on the insulating layer 415. The EL anode layer (transparent electrode) 418 made of a material such as ITO is in contact with the drain area of the TFT 214, and makes up an anode of the organic EL element 216.
Then, an insulating passivation layer 419 such as silicon dioxide is deposited on the surface of the device in the thickness of about 0.5 μm to about 1 μm. The passivation layer 419 is tapered toward an edge 420 on the side of the ITO. The organic EL layer 421 is deposited on the passivation layer 419 and the EL anode layer 418. Finally, the cathode 422 of the organic EL element 216, which is made of a metallic material such as aluminium is deposited on the surface of the device.
The organic EL layer 421 is available in several different structures. For example, the foregoing Tokukaihei 8-234683 discloses a structure of organic EL layer 421 which includes organic hole injection and a moving band in contact with an anode, and electron injection and a moving band for forming a junction with the organic hole injection and the moving band. The Tokukaihei 8-234683 also discloses a structural formula of such an organic layer.
The foregoing circuit operates in the following manner. A voltage for switching ON the TFT 213 is applied to the gate line 151. Then, the TFT 214 is switched ON while accumulating the supplied charge from the source line 152 in the memory capacitor 215. The conduction state of the TFT 214 is also controlled by the stored charged in the memory capacitor 215, even after the TFT 214 is switched OFF, so as to control the current flow through the organic EL element 216.
FIG. 23 shows that the luminance of the organic EL element is nearly proportional to the current. In relation to this, the applied voltage to the organic EL element is related to luminance or luminous efficiency as shown in FIG. 31. FIG. 31 is a graph showing a relation between the applied voltage to the organic EL element, and luminance or luminous efficiency.
The luminous efficiency is determined as follows. Luminance L of the organic EL element and current I through the organic EL element are related byL=A(I)×I(where A(I) is a function which approaches zero when current I is small, and which takes a constant value when current I exceeds a certain value). Further, power consumption W of the organic EL element is related to applied voltage V and current I byW=V×I.Thus, luminous efficiency L/W of the organic EL element isL/W=(A(I)×I)/(V×I)=A(I)/V  (1).
For example, as shown in FIG. 31, the luminous efficiency L/W takes the value 22 [lm/W] at the applied voltage of 3 [V], at which the luminance is 100 [cd/m2]. Further, the luminous efficiency is 15.5 [lm/W] at the applied voltage of 4.4 [V], at which the luminance is 1000 [cd/m2]. This behavior wherein the luminous efficiency L/W once increases with increase in potential V and then decreases can be explained by the function A(I), which shows an abrupt increase with increase in current I up to a certain current I, and which takes almost a constant value above this current I. Further, it can be speculated that the luminous efficiency L/W shows the maximum value in the vicinity of where the function A(I) takes almost a constant value.
In the structure of the simple-matrix-type EL panel, when the number of scanning lines is m, the duration of emission of the organic EL element making up the pixel is only 1/m of the total scanning period. Thus, in order to obtain the same luminance in this pixel as that of a device of a constant-luminescence-type, each duration of emission needs to show spontaneous luminance m times that of the constant-luminescence-type device.
In general, the luminance of white display in laptop personal computers, etc., is around 100 [cd/m2]. Thus, when the number of scanning lines is 100 or more, the required spontaneous luminance for the pixel exceeds 10000 [cd/m2].
However, in the currently available organic EL elements, as shown in FIG. 31, the spontaneous luminance (luminance L) at which the luminous efficiency L/W becomes maximum is around 10 to 100 [cd/m2]. Thus, when such an organic EL element is to be used in a display with the number of scanning lines exceeding 100, the organic EL element needs to be used at low luminous efficiency L/W in the simple-matrix-type EL panel.
In view of this drawback, the active-matrix-type EL panel using diodes as disclosed in the foregoing Tokukaihei 10-268798, and the active-matrix-type EL panel using FETs as disclosed in the foregoing Tokukaihei 8-234683 intended to increase the duration of emission of the organic EL element larger than 1/m of the total scanning period.
However, in the equivalent circuit as shown in FIG. 26, which is the EL panel using diodes, the equivalent circuit will be in the form of an RC serial circuit (resistance-capacitance serial circuit) when the MIM diode 16 becomes non-conducted. Here, the capacitance (capacitance value C) corresponds to the additional capacitance 14 of FIG. 26, and the resistance (resistance value R) presumably corresponds to the sum of the additional resistance 15 (resistance value r) of FIG. 26 and the internal ON resistance of the organic EL element 13 during conduction.
The current I(t) through the organic EL element 13 isI(t)=(qo/RC)exp(−t/RC)  (2)(where t is the elapsed time from the beginning of the non-conduction state of the MIM diode 16, and qo is the charge held in the additional capacitance 14 when time t=0). The charge qo held in the additional capacitance 14 and the voltage Vo generated in the additional capacitance 14 are related byVo=qo/C.
In order to improve luminous efficiency over the simple-matrix-type panel, assuming that the voltage Vo is decided by the withstand voltage of the source driver, it can be seen from equation (1) that the peak value of current I(t) (current I(0)=Vo/R when t=0) needs to be reduced. This requires that the resistance value r of the additional resistance 15 be increased. Note that, since luminance is decided by charge (charge=current value×discharge time at this current value), the luminance will not be changed even when the current is decreased, as long as the discharged charge is constant (because the discharge time is usually short).
However, when the resistance value r of the additional resistance 15 is increased, the time constant RC of the equivalent circuit of FIG. 26 is increased as well. This causes the problem of increased time period for charging the additional capacitance 14. This problem can be solved by increasing the voltage supplied from the source driver, which, however, requires the source driver to have a high withstand voltage, and causes another problem of increased cost of the source driver.
Further, comparing the charging by decreasing the resistance value of the additional resistance 15 and the voltage supplied from the source driver, with the charging by increasing the resistance value of the additional resistance 15 and the voltage supplied from the source driver, the intensity of the current I(t) and the way it flows will be the same between these two methods, provided that the stored charge in the capacitance (capacitance value C) and the charging time are both constant. Since the calorific value of the additional resistance 15 is decided by the product of the square of current and the resistance value, the problem of calorific value in the charging time will be caused as the resistance value is increased.
Further, in this case, the current I(t) through the organic EL element 13 shows an exponential change. Thus, the high luminous efficiency state of the organic EL element 13 cannot be maintained constantly, and further, the low luminous efficiency state may be caused by the change in current I(t). Thus, it is difficult to sufficiently improve the luminous efficiency as with the foregoing case.
Increasing the additional resistance also poses a problem in particular that the generated heat by the current flow through the additional resistance 15 during the charging/discharging period of the additional capacitance C does not contribute to luminescence.
Meanwhile, the active-matrix-type EL panel using TFTs has the following problems. In the active-matrix-type EL panel, the threshold characteristics between the gate and source of the TFT 214 in the equivalent circuit of FIG. 29 do not become uniform within the panel and there is a variance. This variance further causes a variance in a voltage drop between the source and drain, or between the gate and drain, which results in variance in luminance of the organic EL element 216 in the panel.
Further, in this case, the organic EL element 216 is driven by voltage control. Thus, compared with the foregoing case where the organic EL element 216 is driven by current control, luminance becomes instable.
Further, in the case where the organic EL element 216 is displaying tones under the control of the gate voltage of the TFT 214, the current flow through the organic EL element 216 is varied according to the tone level. As a result, the luminous efficiency of the organic EL element 216 is changed according to tones, failing to drive the organic EL element 216 by the current of a range which intends to increase luminous efficiency. Thus, it is also difficult in this case to sufficiently improve luminous efficiency.